1. Field of the Invention
The present invention is concerned with novel bicyclohexyl derivatives, their preparation, liquid crystalline mixtures which contain these compounds and their use for electro-optical purposes.
2. General Discussion
Liquid crystals are used primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to the person skilled in the art and can be based on various effects. Examples of such devices are cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a twisted nematic structure, STN cells ("super-twisted nematic"), SBE cells ("super-birefringence effect") and OMI cells ("optical mode interference"). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have a good chemical and thermal stability and a good stability towards electric fields electromagnetic radiation. Further, the liquid crystals materials should have a low viscosity and in the cells should give short response times, a low threshold potential and a high contrast. Furthermore, at usual operating temperatures of about -30.degree. C. to about +80.degree. C., especially from about -20.degree. C. to about +60.degree. C., they should have a suitable mesophase, for example, a nematic or cholesteric mesophase for the cells referred to above. Further properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfill different requirement depending on the type of cell and field of application. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotrpoy and an electrical conductivity which is as low as possible. For some years there has been a particular interest in actively addressed liquid crystal indicators, for example, TFT applications ("thin film transistor") in television sets. The use of cyano compounds having positive dielectric anisotropy leads, however, in such indicators mainly to an undesirably high increase in current.
Since liquid crystals are generally used as mixtures of several components, it is important that the components have a good miscibility with one another.